I had a similar Idea once with the mii Amiibo but I couldn't quite make it work in my head.
The way it worked is that you got the Amiibo base and several lego-ish (Talking about the lego figurines, not the actual stones) pieces that would make up body and face of the fighter in a base pack.
I thought it could include sticker sheets with Mii facial features to customize look and expression of the Mii as easily as possible.
I also thought that they could have diffrent packs based on the colors of the Miis, but then I ran into the problem that each pack would have to include at least nine bodytypes, to offer sufficient variety, as well as a lot of hairstyles.
Also you would have a lot of leftover parts after building just one miifighter amiibo, wich would just go to waste. So I thought there could be little expansion packs and additional bases available. By that point it had pretty much tured into it's own toyline and I don't think it would be a very viable thing for Nintendo to do! XD
The way I understood your post thoughis not to offer the modular aaproach I was thinking of, but actual custom made Amiibo, wich I think is even less likely to happen, even though I like the idea.
